A solar cell includes a substrate and an emitter layer which are formed of different conductive type semiconductors, such as p-type and n-type semiconductors, respectively. Here, the emitter is placed on a light entering surface of the substrate, and a p-n junction is formed at an interface between the substrate and the emitter.
A front electrode is formed on an upper surface of the emitter to be electrically connected to the emitter, and a rear electrode is formed on the other surface of the substrate opposite the light entering surface to be electrically connected to the substrate.
When light enters such a solar cell, electrons within the semiconductor become free electrons (hereinafter, referred to as “electrons”) through a photoelectric effect, and the electrons and holes move towards the n-type semiconductor and the p-type semiconductor, for example, the emitter and the substrate, respectively, according to the principle of a p-n junction. Then, the electrons and the holes move towards the respective electrodes electrically connected to the substrate and the emitter.
In this solar cell, solar cell efficiency is affected by concentration of a dopant with which the emitter is doped. By way of example, when the dopant with which the emitter is doped is in low concentration, i.e. when the emitter is formed as a lightly doped portion, recombination of electrons and holes is reduced, causing increase in short circuit current density (Jsc) and open circuit voltage (Voc), whereas contact resistance is increased, causing decrease in fill factor. On the contrary, when the dopant with which the emitter is doped is in high concentration, i.e. when the emitter is formed as a heavily doped portion, contact resistance is decreased, causing increase in fill factor, whereas short circuit current density (Jsc) and open circuit voltage (Voc) is decreased.
Thus, recently, there has been developed a solar cell which may take advantage of both the lightly and heavily doped portions, for example, a solar cell including a selective emitter.
A solar cell including a selective emitter has a structure wherein an emitter is composed of a first emitter portion (a lightly doped portion) and a second emitter portion (highly doped portion) and a front electrode is formed on the second emitter portion, and thus exhibits enhanced conversion efficiency as compared with a typical solar cell wherein a dopant is doped at a uniform concentration over the entire area of an emitter.
However, in a solar cell including a selective emitter, when a front electrode is not formed exactly on a selectively heavily doped second emitter portion, parallel resistance is increased, causing reduction in fill factor and thus deterioration in solar cell efficiency.
Therefore, there is a need for an aligned printing method which allows printed electrode patterns to be more accurately bonded to a second emitter portion in printing a composition for electrodes on a selective emitter.